Orexins (hypocretins), a family of hypothalamic neuropeptides, play an important role in modulating feeding behavior, energy homeostasis and the sleep-wake cycle (Siegel, Annu. Rev. Psychol., 55, 125-148, 2004). The orexin-A/hypocretin1 (OX-A, 33 amino acids) and orexin-B/hypocretin2 (OX-B, 28 amino acids) are derived from the same precursor by proteolytic processing of 130 amino acids prepro-orexin (de Lecea et al., Proc Natl Acad Sci USA, 95, 322-327, 1998; Sakurai T. et al., Cell, 92, 573-585, 1998). The orexin levels show a diurnal variation being highest during the active cycle. Two receptor subtypes termed orexin-1 receptor (OX1R) and orexin-2 receptor (OX2R) have been identified. The characterization of both receptors in binding and functional assays demonstrated that OX2R is a non-selective receptor for both OX-A and -B, whereas OX1R is selective for OX-A, conversely OX-A is a non-selective neuropeptide and binds with similar affinities to OX1R and OX2R, while OX-B is selective and has a higher affinity for OX2R (Sakurai T. et al., Cell, 92, 573-585, 1998). Both receptors belong to the class A family of G-protein-coupled receptors (GPCRs) that couple via Gq/11 to the activation of phospholipase C leading to phosphoinositide (PI) hydrolysis and elevation of intracellular Ca2+ levels. However, it has been shown that OX2R could also couple via Gi/o to cAMP pathway (Sakurai, Regulatory Peptides, 126, 3-10, 2005). Northern blot analysis of adult rat tissues showed that the prepro-orexin mRNA is detected exclusively in the brain (except for a small amount in the testis) and that the OX1R and OX2R transcripts are also exclusively detected in the brain (Sakurai T. et al., Cell, 92, 573-585, 1998). Similar results were obtained using human multiple tissue Northern blot. Distribution studies in rat brain using in situ hybridization and immunohistochemistry have shown that orexin neurons are found only in the lateral hypothalamic area with their projections to the entire CNS (Peyron et al., J Neurosci, 18, 9996-10015, 1998; Nambu et al., Brain Res., 827, 243-60, 1999). In addition, both OX1 and OX2 receptors are present in brain regions important for the regulation of sleep/wakefulness.
A disrupted orexin system is suggested to be the cause of narcolepsy based on following lines of evidence: (a) Prepro-orexin knockout mice possessed a phenotype with characteristics remarkably similar to narcolepsy (Chemelli et al., Cell, 98, 437-451, 1999), (b) a mutation (canarc-1), which disrupts the gene encoding OX2R, was found to be responsible for canine narcolepsy (Lin et al., Cell, 98, 365-376, 1999), (c) lack of OX-A and OX-B was observed in human narcoleptic patients (Nishino et al., Lancet, 355, 39-40, 2000; Peyron et al., Nature Medicine, 6, 991-997, 2000), (d) it has been shown that Modafinil, an anti-narcoleptic drug with unknown mechanism of action, activates orexin neurons (Mignot et al., Sleep, 11, 1012-1020, 1997; Chemelli et al., Cell, 98, 437-451, 1999). The intracerebroventricular (icv) administration of OX-A dose-dependently increases wakefulness in rat and also reduces total REM sleep by 84% (Piper et al., Eur. J. Neuroscience, 12, 726-730, 2000). Taken together, these observations are consistent with a crucial role of the orexin system in the modulation of sleep/wake cycle.
Orexin plays an important role in stress and anxiety via its interaction with the corticotropin-releasing factor (CRF) system in hypothalamus (Sakamoto et al., Regul Pept., 118, 183-91, 2004). The icv injection of OX-A induces grooming (stress-response) which is blocked in part by a CRF antagonist (Ida et al., Biochem. Biophys. Res. Comm., 270, 318-323, 2000). OX2R is highly expressed in adrenal medulla, whereas OX1R is high in adrenal cortex. Both OX-A and OX-B stimulate corticosterone release in plasma and induce c-Fos in paraventricular nucleus (PVN) in the hypothalamus (Kuru et al., Neuroreport, 11, 1977-1980, 2000). Furthermore, orexin neurons projecting to CRF neurons express mainly the OX2R (Winsky-Sommerer et al., J. Neuroscience, 24, 11439-11448, 2004). Therefore, OX2R stimulation activates the hypothalamo-pituitary-adrenal (HPA) axis. Interestingly, in this context, the orexin A-induced increases in plasma ACTH has been reported to be attenuated by a selective antagonist to OX-2R (N-{(1S)-1-(6,7-dimethoxy-3,4-dihydro-2(1H)-isoquinolinyl)carbonyl}-2,2-dimethylpropyl)-N-{4-pyridinylmethyl}amine (Chang et al., Neurosci Res., 21 Dec. 2006). A recent preclinical report (Suzuki et al., Brain Research, 1044, 116-121, 2005) has suggested an anxiogenic effect of OX-A. The icv injection of OX-A caused an anxiety-like behavior in mice. Effects were similar to those of corticotropin-releasing factor (CRF) that was tested at the same time for comparison. A recent study has also demonstrated the presence of functional OX1 and OX2 receptors in human adipose tissue and their roles in adipose tissue metabolism and adipogenesis (Digby et al., J. Endocrinol., 191, 129-36, 2006).
In summary, considering the very diverse functions played by orexin system in arousal, sleep/wakefulness, appetite regulation and their roles in anxiety and stress response, etc., one expects that the drugs (or compounds) targeting orexin system will have beneficial therapeutic effects for the treatments of diseases like sleep disorders including sleep apnea, narcolepsy, insomnia, parasomnia, jet lag syndrome, circadian rhythms disorder, restless leg syndrome, psychiatric, neurological and neurodegenerative disorders including anxiety, depression, manic depression, obsessive compulsive disorders, affective neurosis, depressive neurosis, anxiety neurosis, mood disorder, delirium, panic-attack disorder, posttraumatic stress disorders, sexual dysfunction, schizophrenia, psychosis, cognitive disorders, Alzheimer's and Parkinson's diseases, dementia, mental retardation, dyskinesias such as Huntington's disease and Tourette syndrome, addictions, craving associated with drug abuse, seizure disorders, epilepsy, metabolic diseases such as obesity, diabetes, eating disorders including anorexia and bulimia, asthma, migraine, headache pain, neuropathic pain, sleep disorders associated with psychiatric, neurological and neurodegenerative disorders, neuropathic pain, enhanced or exaggerated sensitivity to pain such as hyperalgesia, causalgia, and allodynia, acute pain, burn pain, back pain, complex regional pain syndrome I and II, arthritic pain, post-stroke pain, post-operative pain, neuralgia, pain associated with HIV infection, post-chemotherapy pain, irritable bowel syndrome, extrapyramidal symptoms induced by antipsychotics and other diseases related to general orexin system dysfunction.
Numerous documents describe the current knowledge on orexin pathway, for example the following documents:                Expert Opin. Ther. Patents (2006), 16(5), 631-646        Current Opinion in Drug Discovery & Development, 2006, 9(5), 551-559        J. Neurosci (2000), 20(20), 7760-7765        Neurosci Lett, (2003), 341(3), 256-258        